Multilayer ceramic body



DeC- 14. 1965 J. w. cRowNcvl-:R 3,223,494

MULTILAYER CERAMIC BODY Filed Dec. 5, 1962 United States Patent O3,223,494 MULTILAYER CERAMIC BQDY Joseph Wirt Crownover, San Diego,Calif., assigner to Electro Materials Corporation, La Jolla, Calif.Filed Dec. 3, 1962, Ser. No. 241,977 7 Claims. (Cl. 29--l95) The presentinvention relates to dielectric ceramics and, more particularly to aprocess for producing an electrode for a dielectric ceramic and theelectrode produced thereby.

In recent years, dielectric ceramics have achieved substantialacceptance in the electronic industry as a basic element in theproduction of small capacitors. A sheet of a dielectric ceramic iselectroded and encapsulated with leads protruding to provide a smallunitary structure which can be readily incorporated into small highfrequency electronic circuits. At high frequency, capacitances in therange of micro-microfarads or picofarads are not uncommon and it hasbeen found that dielectric ceramics provide these capacitances withgreat efficacy.

In the prior art, problems have been encountered in finding suitableelectrode materials and in linding suitable methods for aiiixingelectrodes to the ceramic dielectric which forms the basis of thecapacitor. Typically, electrodes are soldered, welded, or otherwiseattached to the ceramic, all of which are relatively expensive since theoperation requires handling of the dielectric. Moreover, theseelectrodes may, under suitable environmental conditions, become detachedfrom the ceramic body thereby changing the capacitance with undesirableresults.

Small capacitors of higher values of capacitance have been attempted bystacking alternate layers of electrode and ceramic, and interconnectingalternate electrodes to produce a plurality of paralleled capacitors.However, this combination was diicult to fabricate and lacked mechanicalrigidity, and was costly because of the need for handling the individuallayers.

One promising approach discovered by the prior art researchers was thecompounding of an electroding composition from the same ceramic materialused in the dielectric. Some 40 to 60% by weight of a metallic powder orits oxide was added to the raw slurry and the mixture was fired in areducing atmosphere. Such a process tends to be rather intricate andrequires expensive, special equipment. For example, one suggestedreducing atmosphere was hydrogen gas which requires a special explosionproof furnace.

According to the present invention, a slurry consisting of raw, unredceramic includes a plastic or resin binder that is maintained in theliquid state by the addition of substantial amounts of a relativelyvolatile solvent. This slurry is then cast and the solvent is allowed toevaporate, leaving a relatively tiexible, plastic, green ceramic ware.

An electroding material is made up by using this same ceramic slurry towhich has been added a heavy concentration of a highly conductive metal,such as platinum in powder form. The electroding composition can then besprayed, coated, or otherwise applied to the surfaces of the greenceramic and the resulting combination is then fired. In the firingprocess, the resin vehicle volatilizes, leaving the ceramic behind.During ring, the electrode layers coalesce into the ceramic body,forming a unitary, relatively homogeneous structure, which is conductiveat the surfaces and is a dielectric in the center portion.

In a preferred embodiment, the electroding composition containsapproximately 85% metal, by weight, to by weight, of ceramic. Moreceramic tends to reduce the conductivity and more metal powder reducesthe structural strength.

" Multi-layer ceramic capacitors are produced by stack- 3,223,494Patented Dec. 14, 1965 ing layers of ceramic and electrode, which areprepared by coating ceramic sheets with electrode composition on oneside and then stacking the sheets. Alternatively, single sheets areelectroded on both surfaces and then are accordion-folded, resulting ina plurality of layers of dielectric ceramic separated by layers ofelectrode.

The novel properties of the electroding material permit interestingalternative structures within the present invention. Multiple dielectriclayers, separated by electrode layers can be fired into a multi-layer,monolithic structure having higher values of capacitance.

It is therefore an object of the present invention to provide animproved process for electroding dielectric ceramics.

It is an additional object of the invention to provide an improvedelectrode for delectric ceramics.

It is a further object of the invention to provide an improveddielectric capacitor.

It is yet another object of the invention to provide a process for theproduction of multi-layer ceramic dielectric capacitors.

It is still a further object of the present invention to provide animproved multi-layer, dielectric ceramic capacitor.

The novel features which are believed to be characteristic of theinvention both as to its organization and method of operation togetherwith further objects and advantages thereof will be better understoodfrom the following description considered with the accompanying drawingsin which several embodiments are illustrated by way of example. It isspecifically understood, however, that the drawings are for the purposeof illustration and description only and are not intended as adefinition of the limits of the invention.`

FIG. l is an end sectional view of a dielectric ceramic with electrodesapplied according to the present invention.

FIG. 2 is an end sectional view of a dielectric ceramic layer with anelectrode applied to one surface thereof.

FIG. 3 comprises FIGS. 3a-3c of which:

FIG. 3a is an end sectional view of an alternative electrodeconfiguration;

FIG. 3b is an end View of the layer of FIG. 3a cut along the line 3 3and the halves stacked;

FIG. 3c is an end sectional view of a stack of dielectric layers;

FIG. 4 comprises FIGS. 4a and 4b, of which:

FIG. 4a is an end sectional view of yet another electrode configuration;

FIG. 4b is an end view of the layer of FIG. 4a cut along line 4-4 andstacked;

FIG. 5 comprises FIGS. 5a, 5b and 5c of which:

FIG. 5a is an end sectional view of a dielectric ceramic layer to whichelectrodes are applied on both upper and lower surfaces;

FIG. 5b is a side sectional view of the dielectric ceramic of FIG. 5aalong the line 5 5 in the direction of the appended arrows, in which theceramic layer is accordionfolded to form a multi-layer dielectricceramic, similar to that of FIG. 3c; and

FIG. 5c is a side View of the ceramic of FIG. 5b completely folded andattened.

In practicing the present invention, there are many acceptableformulations for the dielectric ceramic all of which are suitable foruse as capacitors. One such formulation is a mixture of barium titanate[BaTiO3] and bismuth stannate [Bi2(SnO3)3] in proportions of 96.4 molpercent and 3.6 mol percent, respectively.

These ingredients are prepared in a ball mill and are mixed with avehicle of methyl methacrylate, sometimes known by its trademark name,Plexiglas, which is made iiuid by the addition of a suitable solvent,such as ethylene dichloride.

This mixture thus formed of the ceramic and the vehicle is sometimescalled a slurry which may be cast. When permitted to dry at a slightlyelevated temperature, for example, 200 F. for a suitable period of time,depending upon the thickness of the layer, a dry flexible ceramic sheetalso known as ware, is formed which can be easily handled. To aid inuniform drying, a vacuum may be employed to draw oi the solvent.

Using this same ceramic slurry as a starting material, an electrodingcomposition is formed by adding a highly conductive metal in powderform. One preferred metal is platinum, which is readily available as apowder. Finely powdered silver also may be used, as may any other highlyconductive metal. According to the present invention, thev electrodecomposition can range from 80 to 92% metal by weight, with the remainderbeing the ceramic.

In the preferred formulation, the ratio is 85% metal to ceramic. Theresistivity of the electrode increases appreciably if less thanV 80% ofthe mixture is metal. If the electroding composition contains less than8% of ceramic, then the composition will lack cohesion and structuralstrengthV after tiring, and would tend to crumble.

After the dielectric ceramic is cast, the electroding material can beapplied to the surfaces either by dipping, painting, spraying, orthrough silk screen printing techniques. The electroding composition isalso permitted to dry, preferably at an elevated temperature in avacuum. The electroded ceramic is then fired in a kiln in which thetemperature is raised in two hours, to 2450 F. This temperature is heldfor one hour and the kiln is then permitted to cool slowly forapproximately five hours, during which the temperature is reduced at arate of 200 per hour. At the'end of the five hour period, the ceramic istaken from the kiln and permitted to cool down to room temperature.

The resulting product can be diced into small squares or rectangleswhose capacitance is substantially proportional to the area.

Turning now to FIG. 1there is shown a typical dielectric ceramic plate10 with electrodes applied thereto. By aflxing conductive leads to theelectrode surfaces and encapsulating the structure, a ceramic capacitoris produced. As shown in the figure, a dielectric ceramic portion 12 iscoated on the upper and lower surfaces with an electrode material 14.

lFor special applications in which more than one dielectric layer isused in the nished capacitor, an alternative electrode configuration isdesirable. In FIGURE 2, there is shown one such conguration wherein theelectrode material 14 is applied to substantially all of the surface ofthe dielectric ceramic 12. A strip along one of the edgesy is `leftbare. If a plurality of such plates are provided-with the electrodelayer of successive layers abutting, then a stack could be fabricatedwith additional electroding composition 14 applied to the opposite edgesof the stack to create a parallel electrical connection of theelectrodes of alternate layers.

VTurning now to FIG. 3a, there is shown an alternativeelectrode'conguration which is particularly suited to the making ofmulti-layer stacks. As seen in FIG. 3a, the dielectric ceramic 12 hastwo strips of electrode 14 applied to the upper surface, separated by anuncoated strip, If now the ceramic layer is cut along line 3 3, thehalves can be stacked as shown in FIG. 3b, and with the addition ofother layers, a multi-layer stack, such as is shown in FIG. 3c results.It will, of course, be apparent to those skilled in the art, that thefigures are exaggerated to show detail and that the scale is oversize.

In FIG. 3c, the application of additional electrode material 14 to theedges is illustrated which provides the parallel interconnection ofalternate electrodes. After tiring, all apparent cavities will be lledby the owing of the ceramic 12 to form a substantially homogeneous mass.

FIG. 4a shows a similar electrode pattern which can form the basis of astack after a single layer is cut down the middle along line 4 4. FIG.4b illustrates the stacking of the resultant halves.

Still another electroding method can be used to produce multi-layerstacks. Turning to FIG. 5a, there is shown a single layer of dielectricceramic 12 to which has been applied upper and lower electroding layers14. As seen in FIG. 5a, the upper layer abuts only the left hand edge asViewed in the figure while the layer abuts only the right hand edge. InFIG. 5c, the ceramic layer is shown after the strip has been folded inan accordion fashion to form the beginning of a multi-layer stack. Thenished stack is shown in FIG. 5c, in which adjacent layers of dielectric12 are separated by double thicknesses of electrode 14. While still inthe green or unred state, the stack can be trimmed at the folds so thata substantially flat, horizontal stack results. The edges are coatedwith electroding material 14 to connect the alternate electrodes inparallel.

Thus, there has been described an improved method of providing anelectroding compound f-or a dielectric ceramic. Applying electrodesaccordi-ng to the present invention is easily susceptible of massproduction techniques and there is no requirement that each of thedielectric ceramic elements be handled individually in order to haveelectrodes applied thereto. Rather, electrodes can be applied before theware is red and the resultant ceramic may be subsequently cut into thedesired sections of suitable size and capacitance. Each dielectricsegment then need only have leads affixed thereto before encapsulation.

Also, a new multi-layer dielectric ceramic of monolithic structurehaving unusual structural uniformity and homogeneity has been shownwhich is easily fabricated in mass production. Multi-layer stacks, whenred, become unitary bodies which are inseparable. By applying theelectroding mixture of platinum and ceramic to the exposed outer layersonly, some cost savings may be achieved by using a silver-ceramicmixture on all of the interior electrode layers. However, silver tendsto disappear at the curing temperatures used when applied to the exposedouter layers, but in the internal, sealed layers, this is not a problem.

Adopting empirical rules, it would seem that satisfactory electrodingmixtures, at least for the sealed, interior layers, could be preparedfrom the ceramic and any metal whose resistivity at normal temperaturesis less than 10 microohm-centimeters and whose boiling point is wellabove the maximum temperature achieved during the ring cycle. Exteriorlayers would probably require the metal to have a relatively low vaporpressure to prevent evaporation at temperatures below boiling but abovemelting.

The metals should not oxidize in the presence of oxygen, titanates, andstannates during the curing but should remain as metals throughout theprocess.

Although they have not yet been tried, chromium, cobalt, iridiurn, iron,molybdenum, nickel, rhodium and tungsten appear to be useful inelectroding mixtures. llfheir melting points are all higher than themaximum curing temperatures recommended and their resistivity is l0microhm-centimeters or less.

Other variations will be readily apparent to those skilled in the artand the scope of the invention should be limited only by the scope ofthe claims appended hereto.

What I claim as new is:

1. A multi-layer dielectric ceramic body suitable for use as acapacitive impedance element comprising: alternate strata of adielectric ceramic material of predetermined formulation and a mixtureof from 20% to 8% by weight of said dielectric ceramic material ofpredetermined formulation combined with from to 92% by weight of a metalof high electrical conductivity.

2. The multi-layer dielectric ceramic body 0f claim I in which saidmetal is platinum.

3. The multi-layer dielectric ceramic body of claim 1 in which saidmetal is silver.

4. The multi-layer dielectric ceramic body of claim 1 in which saidmetal is chosen from the group consisting of chromium, cobalt, iridium,iron, molybdenum, nickel, platinum, rhodium, and tungsten.

5. A multilayer ceramic body suitable for use as a capacitor comprising:

a central electrically non-conductive layer of a high dielectric ceramicof predetermined formulation; and

top and bottom electrically conductive layers of said high dielectricceramic of predetermined formulation into which has been mixed a metalpowder of high electrical conductivity in the proportions of 8% to 20%by weight of said ceramic and 92% to 80% by weight of said metal powder.

6. The multilayer ceramic body of claim 5 in which the proportions ofsaid ceramic to metal in said top and bottom layers are 5% ceramic to85% metal.

7. The multilayer ceramic body of claim 6 in which said metal powder isplatinum.

References Cited by the Examiner UNITED STATES PATENTS DAVID L. RECK,Primary Examiner. HYLAND BIZOT, Examiner.

1. A MULTI-LAYER DIELECTRIC CERAMIC BODY SUITABLE FOR USE AS A CAPACITIVE IMPEDANCE ELEMENT COMPRISING: ALTERNATE STRATA OF A DIELECTRIC CERAMIC MATERIAL OF PREDETERMINED FORMULATION AND A MIXTURE OF FROM 20% TO 8% BY WEIGHT OF SAID DIELECTRIC CERAMIC MATERIAL OF PREDETERMINED FORMULATION COMBINED WITH FROM 80% TO 92% BY WEIGHT OF A METAL OF HIGH ELECTRICAL CONDUCTIVITY. 